Standalone service along with an AppleID / Apple Account

[Mity]

My dad currently has an Android phone with service from T-Mobile Prepaid. I recently bought him an Apple Watch with standalone service from T-Mobile Postpaid. If I get him an iPhone, which will have mobile service on Prepaid, can he still use his Apple Watch using standalone service from Postpaid, even though they would both be on the same AppleID / Apple Account?

 

[Bigwaff]

If I get him an iPhone, which will have mobile service on Prepaid, can he still use his Apple Watch using standalone service from Postpaid, even though they would both be on the same AppleID / Apple Account?

Yes. Cellular plans have nothing to do with Apple Account.

 

[FreakinEurekan]

He could, yes. However he'll get a lot more functionality if he pairs it to his iPhone. Calls and messages on the watch & phone, for starters, but also a LOT of apps that don't work on a Family watch.